This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The main purpose of this study is to find out what doses of the study drug are safe for people with SCD and if the study drug helps with endothelial dysfunction &#40;problems with the lining of the arteries&#41;. The lining of arteries &#40;blood vessels that carry blood and oxygen from the heart to the rest of the body&#41;is made up of smooth flat cells, called endothelial cells. Other substances in the body, such as tetrahydrobiopterin &#40;BH4&#41;, help the body make the right amount of NO. If there is not enough BH4 in your body &#40;Called BH4 deficiency&#41;, then you may not have enough NO to help the arteries relax. If this happens, then not enough blood and oxygen flows through the blood vessels. This is sometimes called endothelial dysfunctions. This means that the endothelial cells do not work like they should.